- The big picture[[Student version, January 17, 2003]] 291
chain, can now find and bind to those on other chains. In this way a crosslinked network of chains
can form. The interstices of this network can hold water, and the result is a solid gel: the cooked
egg. As with milk, one may expect that the addition of acid would enhance the coagulation of eggs
once the proteins are denatured, and indeed it’s so.
Heating is not the only way to denature egg proteins and create a linked network. Merely
whipping air into the eggs, to create a large surface area of contact with air, can totally disrupt the
hydrophobic interactions. The ensuing “surface denaturation” of egg proteins like conalbumin is
what gives chiffon pie or mousse their structural stability: A network of unfolded proteins arrange
themselves with their hydrophobic residues facing the air bubbles, while their hydrophilic ones face
the water. This network not only reduces the air-water tension like any amphiphile (Section 8.4.1);
it also stabilizes the arrangement of bubbles, since unlike simple amphiphiles the proteins are long
chains. Other proteins, like ovomucin and globulins, play a supporting role by making the egg
viscous enough that the initial foam drains slowly, giving the conalbumin time to form its network.
Still others, like ovalbumin, support air foams but require heat for their initial denaturation; these
are key to supporting the stronger structures of meringue and souffl ́e. All of these attributions of
specific roles to specific proteins were established by isolating particular proteins and trying them
alone or in various combinations.
Eggs also serve as emulsifiers, for example in the preparation of creamy sauces as mentioned
earlier. Preparing such sauces is tricky; a slight deviation from the recipe can turn a nice emulsion
into a coagulated mess. Immense volumes of superstition and folklore have evolved on this subject,
concerning clockwise versus counterclockwise stirring and so on. Most of these claims have not
survived scientific scrutiny. But in a careful study, a group of high-school students found that
simply adding lecithin, a two-chain phospholipid available at health-food stores, can reliably rescue
afailed sauce B ́earnaise, an unsurprising conclusion in the light of Section 8.4.1.
The big picture
Returning to the Focus Question, an activation barrier can lock energy into a molecular species,
making the release of that energy negligibly slow. A beaker with a lot of that species may not be
doing much, but it is far from equilibrium, and hence ready to do useful work. We saw how to make
such statements quantitative, using the change of free energy when a single molecule enters or leaves
asystem (the chemical potentialμ). We got a formula forμshowing the reciprocal roles of energy
and entropy in determining the chemical force driving chemical reactions, and unifying chemical
forces with the other sorts of entropic forces studied in this book. Chapters 10–11 will extend our
understanding from chemical reactions tomechanochemical andelectrochemical reactions, those
doing useful work by moving an object against a load force or electrostatic field. Such reactions,
and the enzymes that broker them, are central to the functioning of cells.
Key formulas
- Chemical potential: Suppose there are Ω(E, N 1 ,N 2 ,...)states available to a system with
energyE andN 1 particles of type 1, and so on. The chemical potential of speciesα is
thenμα=−T∂N∂Sα|E,Nβ,β=α(Equation 8.1). μαdescribes the “availability of particles” for
exchange with another subsystem. If each of theμαfor one system agree with the corre-
sponding values for the other, then there will be no net exchange (Equation 8.2).